US7882896B2 - Gas eduction tube for seabed caisson pump assembly - Google Patents

Abstract

A seafloor pump assembly is installed within a caisson that has an upper end for receiving a flow of fluid containing gas and liquid. The pump assembly is enclosed within a shroud that has an upper end that seals around the pump assembly and a lower end that is below the motor and is open. An eduction tube has an upper end above the shroud within the upper portion of the caisson and a lower end in fluid communication with an interior portion of the shroud. The eduction tube causes gas that separates from the liquid and collects in the upper portion of the caisson to be drawn into the pump and mixed with the liquid as the liquid is being pumped.

Description

FIELD OF THE INVENTION

This invention relates in general to pumping well fluid from the seabed to the surface, and in particular to a pump assembly located within a caisson and having an eduction tube to reduce gas accumulation in the caisson.

BACKGROUND OF THE INVENTION

Offshore wells are being drilled in increasingly deeper waters. The wells may have adequate pressure to flow the well fluid to the seabed, but lack sufficient pressure to flow the fluid thousands of feet upward to a production vessel. Proposals have been made to install pumps at the seabed to boost the pressure of the well fluid sufficiently to flow it to the floating production vessel.

Often, the well fluid will be a mixture of hydrocarbon liquid, gas and water. Gas presents a problem for pumps, particularly electrically driven centrifugal pumps. Gas detracts from the efficiency of the pump, and can cause the pump to lock and shut down if a large slug of gas enters.

One proposal for dealing with well fluid having an appreciable quantity of gas is to mount the pump in a caisson. The caisson is located in a tubular bore formed into the seabed and cased to seal it from the earth formations. The caisson may be several hundred feet deep. The well fluid flows in the upper end of the caisson, and gravity causes the liquid to separate from the gas and flow downward in the caisson. The gas tends to collect in the upper portion of the caisson. The submersible pump is located within the caisson at a point where its intake is below the liquid level. The pump is enclosed by a shroud with an inlet at the lower end to force liquid to flow upward by the motor to coot the motor. As the gas cap continues to build, portions will escape and flow into the pump along with the liquid to be pumped into the surface. A possibility exists that the gas cap will grow and push the liquid level too low, resulting in a large quantity of the gas entering the pump and causing it to gas lock. Liquid level controllers have been proposed to open and close the inlet to the caisson to try to maintain the liquid at a desired level above the intake of the pump. A large gas slug could nevertheless still enter the pump and cause a gas lock.

SUMMARY OF THE INVENTION

In this invention, the pump is located within a shroud inside the caisson. An eduction tube that extends out of shroud and has an upper end for location within a portion of the caisson that normally will be a gas accumulation area above the liquid level. The eduction tube has a lower end in fluid communication with an interior portion of the shroud. During operation, the eduction tube creates a suction to draw in a small continuous quantity of gas as the pump operates to avoid the gas cap from becoming too large.

In one embodiment, the lower end of the tube joins the intake of the pump assembly within the shroud. In another embodiment, the eduction tube extends alongside the shroud and has its lower end at the inlet of the shroud. Preferably the inlet of the shroud in that instance has a venturi configuration to cause a reduced pressure. The lower end of the tube joins a point of reduced pressure in the venturi.

In another embodiment, more than one eduction tube is employed. The tubes may have their upper ends spaced at different distances above the shroud for educting gas from different points in the caisson. In another embodiment, an eduction conduit is mounted to the inlet of a caisson. The eduction conduit leads from the upper end of the caisson back to the inlet for recirculating some of the gas cap back into the well fluid flowing into the caisson. In all of the embodiments, the eduction tube or tubes are sized to have a much smaller flow area than the flow area of the inlet of the shroud, so that significant amount liquid will continue to flow into the inlet of the shroud.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating a caisson pump apparatus constructed in accordance with a first embodiment of the invention.

FIG. 2 is a schematic sectional view of a caisson pump apparatus constructed in accordance with a second embodiment of the invention.

FIG. 3 is a schematic sectional view of a caisson pump apparatus constructed in accordance with a third embodiment of the invention.

FIG. 4 is a schematic sectional view of a caisson pump apparatus constructed in accordance with a fourth embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, acaisson11 is shown schematically. Caisson11 comprises a hole that has been formed in the seafloor to a desired depth, which may be several hundred feet. Caisson11 is encased in a casing that is impermeable to any fluids fromearth formation15. Caisson11 has aninlet13 that is located near its upper end, such as slightly above the seabed.

Ashroud17 is located within caisson11. Shroud17 has aninlet19 at its lower end, Shroud17 is a tubular member that is smaller in diameter than the inner diameter ofcaisson11 so as to create an annular passage surrounding it for downward fluid flow.

An electrical submersible pump assembly (“ESP”)21 is mounted withinshroud17.ESP21 has apump23 that is typically a centrifugal pump.Pump23 is made up of a large number of stages, each having a rotating impeller and a stationary diffuser.Pump23 has anintake25 that is located at the lower end ofpump23 withinshroud17. Shroud17 has anupper end27 that seals around a portion ofESP21 aboveintake25. If desired, the entire length ofESP21 could be enclosed byshroud17, but theupper end27 ofshroud17 only needs to be slightly abovepump intake25. Adischarge pipe29 extends upward frompump23 and out the upper end ofcaisson11. Although shown extending through the top ofcaisson11,discharge pipe29 could alternately extend through a sidewall portion ofcaisson11.ESP21 also has anelectrical motor31 that has a shaft that drivespump23.Motor31 andpump23 are conventionally separated by aseal section33.Seal section33 equalizes the pressure of lubricant contained inmotor31 with the well fluid on the exterior ofmotor31.

Aneduction tube35 has anupper end37 that is exterior ofshroud17.Eduction tube35 has an inner diameter much smaller than the inner diameter ofdischarge pipe29.Eduction tube35 has alower end39 that is fluid communication with well fluid in the interior ofshroud17. In the first embodiment,lower end39 extends to a portion ofpump intake25. Whenpump23 is operating, a suction exists atintake25, causinglower end39 to have a lower pressure thanupper end37.Upper end37 is positioned above theliquid level40 incaisson11 at all times. Optionally, a liquid level controller (not shown) may employed for controlling the flow of fluid intocaisson11, if desired, to maintainliquid level40 fairly constant.

In the operation of the first embodiment,ESP21 is placed inshroud17 and installed incaisson11. The valve (not shown) toinlet13 is opened, causing well fluid to flow throughcaisson inlet13. The well fluid is typically a mixture of hydrocarbon liquid, water and gas.Shroud17 is immersed in liquid incaisson11, withliquid level40 being at least abovepump intake25 and preferably above shroudupper end27.Liquid level40 will be belowcaisson inlet13. A gravity separation occurs as the fluid flows ininlet13 and downward incaisson11. This results in gas freeing from the liquid and collecting in the upper portion ofcaisson11. The liquid flows down through the annular passage aroundshroud17 and intoshroud inlet19. The liquid flows up alongsidemotor31 and intopump intake25.Pump23 increases the pressure of the liquid and discharges it throughdischarge pipe29 for flowing the liquid to the surface.

At the same time, a small amount of gas from the gas cap collecting aboveliquid level40 will flow througheduction tube35. The gas leaveseduction tube35 and mixes with the liquid flowing intopump intake25. The flow rate of the gas is fairly constant and relatively small compared to the liquid flow rate, thus is readily pumped bypump23 along with the liquid updischarge pipe29. The flow area ofeduction tube35 is much smaller than the total flow area ofshroud inlet19 so as to avoid excessive amounts of gas flowing intopump23. Also, the small cross-sectional flow area ofeduction tube35 assures that liquid will continue to flow up aroundmotor31 for coolingmotor31.

In the embodiment ofFIG. 2, the components that are the same as inFIG. 1 have the same reference numerals.Shroud41 differs fromshroud17 in that its inlet comprises aventuri43.Venturi43 has a converging lower orupstream section45 that joins a throat orcentral section47 of reduced but constant diameter.Central section47 leads to a downstream divergingsection49.Venturi43 causes a reduced pressure incentral section47.Eduction tube51 has itsupper end53 positioned aboveshroud41, as in the first embodiment. Thelower end55 ofeduction tube51 joins venturicentral section47.

The second embodiment operates in the same manner as the first embodiment by drawing a portion of the gas cap continuously down througheduction tube51 intoshroud41. In this embodiment, the gas mixes with the liquid as it flows upward aroundmotor31 and intopump intake25.

In the embodiment ofFIG. 3, asecond eduction tube57 is employed along withfirst eduction tube53.Eduction tube57 also extends alongsideshroud41 and has its lower end connected with venturicentral section47.Second eduction tube57 increases the amount of gas being drawing from the gas cap.Second eduction tube57 may have its upper end at a different elevation fromfirst eduction tube51, if desired. This results insecond eduction tube57 drawing gas from a different portion ofcaisson11. If the liquid level rose to a point above second eduction tubeupper end59, first eduction tubeupper end53 might be high enough to continue drawing gas.Second eduction tube57 is shown added to the embodiment ofFIG. 2. Alternatively, it could be added to the embodiment ofFIG. 1 as well.

In the embodiment ofFIG. 4, acaisson venturi61 is provided at the inlet ofcaisson11.Caisson venturi61 has anupstream section63 that converges, a central orthroat section65 of smaller diameter, and adownstream section67 that diverges. Aneduction conduit69 has aninlet end71 connected to an upper portion ofcaisson11. The outlet ofeduction tube69 is located at venturicentral section65.

In the operation of the embodiment ofFIG. 4, the well fluid flowing intocaisson11 is conditioned byventuri61 in that gas collecting in the upper portion ofcaisson11 will be metered back into the fluid flowing throughcaisson venturi61 to re-entrain the gas in the fluid flow. The gas will be dispersed into smaller bubbles as it is re-entrained. The smaller bubbles are more readily pumped bypump23 than large slugs of gas.Caisson venturi61 causes a pressure drop that recirculates some of the accumulated gas back into the incoming liquid stream. Although the embodiment ofFIG. 4 is shown as containing thesame eduction tubes51,57 as inFIG. 3, theFIG. 4 embodiment could also be employed with theFIG. 1 or theFIG. 2 embodiments.Eduction conduit69 can condition the gas in the well fluid in all three of the embodiments.

The invention has significant advantages. By continuously drawing off a small amount of the gas cap, the size of the gas cap is maintained within the caisson at a minimum dimension. Limiting the size of the gas cap prevents the liquid level from dropping so low that such large slugs of gas could enter the shroud and cause gas locking of the pump.

While the invention has been shown in only a few of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention.

Claims (20)

1. A sea floor pump apparatus for installation in a caisson having an inlet at an upper end for receiving a flow of fluid containing gas and liquid, the apparatus comprising:

a shroud for location within the caisson, the shroud having an inlet at a lower end for receiving fluid flowing into the caisson;

an electrical submersible pump assembly mounted within the shroud, the pump assembly including a centrifugal pump having an intake within the shroud downstream of the inlet of the shroud for drawing fluid from an interior of the shroud into the pump, the pump having a discharge pipe for discharging the fluid out of the caisson; and

at least one eduction tube having an open upper end extending exterior of the shroud for location within an upper portion of the caisson and an open lower end in fluid communication with an interior portion of the shroud at or below the intake of the pump for drawing gas that collects in the caisson into the intake of the pump.

2. The apparatus according toclaim 1, wherein the lower end of the tube extends through a wall portion of the caisson and joins the intake of the pump assembly.

3. The apparatus according toclaim 1, wherein the tube extends alongside the shroud and has its lower end at the inlet of the shroud below the intake of the pump.

4. The apparatus according toclaim 1, wherein the inlet of the shroud comprises:

a venturi having an upstream converging portion, a central portion of reduced diameter, and a downstream diverging portion; and wherein

the lower end of the tube extends to the central portion of the venturi.

5. The apparatus according toclaim 1, wherein the upper end of the tube is located above the shroud.

6. The apparatus according toclaim 1, wherein said at least one tube comprises:

first and second tubes, the first tube having an upper end at a higher elevation from the shroud than the second tube.

7. The apparatus according toclaim 1, wherein the tube has a flow area substantially smaller than a flow area of the inlet of the shroud.

8. The apparatus according toclaim 1, wherein the tube has an inner diameter substantially smaller than an inner diameter of the discharge pipe.

9. A sea floor fluid pump apparatus, comprising:

a caisson installed in a sea floor and having an inlet at an upper end for receiving a flow of fluid containing gas and liquid;

an electrical submersible pump assembly within the caisson, the pump assembly including a centrifugal pump having an intake and a discharge pipe extending sealingly through an upper portion of the caisson;

a shroud surrounding the pump assembly within the caisson, the shroud having an upper end sealed around the pump assembly above the intake of the pump and an inlet at a lower end for receiving fluid flowing into the caisson, the downward flow of the fluid around the shroud causing at least some of the gas contained therein to separate and collect in an upper portion of the caisson, the intake of the pump being in fluid communication with an interior of the shroud; and

at least one tube having an open upper end above the shroud within the upper portion of the caisson and an open lower end in fluid communication with an interior portion of the shroud for drawing gas that collects in the caisson into the intake of the pump, the lower end of the tube being at an elevation no higher than the intake of the pump.

10. The apparatus according toclaim 9, wherein the lower end of the tube extends through a wall portion of the shroud and is coupled directly to the intake of the pump above the inlet of the shroud.

11. The apparatus according toclaim 9, wherein the tube extends alongside the shroud and has its lower end at the inlet of the shroud.

12. The apparatus according toclaim 9, wherein the inlet of the shroud comprises:

a venturi having a lower converging portion, a central portion of reduced diameter, and an upper diverging portion; and wherein

the lower end of the tube extends to the central portion of the venturi.

13. The apparatus according toclaim 9, wherein said at least one tube comprises:

first and second tubes, the first tube having an upper end at a higher elevation within the caisson than the second tube.

14. The apparatus according toclaim 9, wherein the tube has an inner diameter substantially smaller than the discharge pipe of the submersible pump assembly.

15. The apparatus according toclaim 9, wherein the tube has a flow area substantially. smaller than a flow area of the inlet of the shroud.

16. The apparatus according toclaim 9, wherein the inlet of the caisson comprises:

a venturi having an upstream converging portion, a central portion of reduced diameter, and a downstream diverging portion; and wherein

a recirculation conduit extends from the central portion of the venturi to the upper portion of the caisson for educting gas collecting in the upper portion of the caisson and re-entraining the gas with the fluid flowing into the caisson.

17. A method of pumping a well fluid from a sea floor, comprising:

(a) providing a caisson in the sea floor;

(b) mounting a shroud around an electrical submersible pump assembly that includes a centrifugal pump having an intake within the shroud, the shroud having an inlet at a lower end of the shroud;

(c) connecting at least one tube to the shroud such that an upper end of the tube is exterior of the shroud and the lower end of the tube is in fluid communication with an interior portion of the shroud at or below the intake of the pump;

(d) installing the shroud, the pump assembly and the tube in the caisson;

(e) flowing a fluid containing a gas and liquid into the caisson and operating the pump assembly, causing the fluid to flow downward around the shroud and up the inlet of the shroud into the intake of the pump, which discharges the fluid out of the caisson at a greater pressure, the downward flow of the fluid in the caisson causing some of the gas to separate and collect in an upper portion of the caisson; and

(f) educting gas collected in the upper portion of the caisson through the tube into the shroud and into the intake of the pump.

18. The method according toclaim 17, further comprising:

maintaining a level of the liquid in the caisson below the upper end of the tube.

19. The method according toclaim 17, further comprising:

educting a portion of the gas collected in the upper portion of the caisson back to an inlet of the caisson and re-entraining the gas with the fluid flowing into the caisson.

20. The method according toclaim 17, wherein step (c) comprises:

positioning the upper end of the tube above the shroud.

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